Photosensitive resin compositions are classified into A) polarity-changing type wherein the polarity of the exposed regions is changed so that the solubility thereof is changed, B) cutting type wherein chemical bonds are cut by exposure and the exposed regions are solubilized, and C) cross-linking type wherein cross-linking reaction proceeds so that exposed regions are insolubilized. The polarity-changing type may be used as either positive-type or negative-type composition depending on the composition of the developing solution. The cutting type may be used as positive-type composition, and the cross-linking type may be used as negative-type composition in theory. The cross-linking type photosensitive materials have a disadvantage in carrying out microscopic processing with high resolution that the exposed regions are swollen by the developing with an organic solvent.
In recent years, the molding materials used for overcoating flexible printed circuits, interlayer insulation films of multilayer substrates, insulation films and passivation films of solid elements in semiconductor industry, as well as the interlayer insulation materials of semiconductor integrated circuits and multilayer printed circuit boards are demanded to have good heat resistance. Further, the need to attain higher densification and higher integration demands photosensitive heat-resisting materials.
The semiconductor substrates which are used as semiconductor integrated parts in microelectronic industry are covered with photoresists. Photoresist relief structures are formed by forming images and subsequent development of the photoresist layers. The relief structures are used as the masks for preparing circuit patterns on the semiconductor substrates. By this processing cycle, the relief structure of a microchip can be transferred to a substrate.
Photoresists include two different types (positive-type photoresist and negative-type photoresist). These are different in that the exposed regions of the positive-type photoresist is removed by development so that the non-developed regions are left on the substrate, while the exposed regions of the negative-type photoresist are left as the relief structure. The positive-type photoresists have intrinsically high image resolutions so that they are suited for production of VLSIs (large scale integrated circuits).
Conventional positive-type photoresists contain a type of novolak resin which is soluble in aqueous alkali and a photosensitive quinone diazide which decreases the solubility of the resin in alkali. When the photoresist layer is irradiated, the quinone diazide is photoexcited so as to be converted to carboxylic acid, so that the solubility in alkali of the exposed regions is increased. Thus, by developing the photoresist with an aqueous alkali, a positive-type photoresist relief structure is obtained (USP 36664735 etc).
The characteristics of the photoresist compositions used in industries are the solubility of the photoresist in the solvent to be applied, the photosensitization rate of the photoresist, the developing contrast, the solubility of the developing solution acceptable from the view point of environment, adhesiveness of the photoresist, dimensional stability at high temperatures, and abrasion resistance.
The photoresist relief structures obtained by the exposure and development are usually subjected to heat treatment (postbake) at a temperature of 120° C. to 180° C. The purpose of this treatment is to promote the adhesiveness of the photoresist with the substrate, curing of the photoresist structure, and removal of all of the remaining volatile components to decrease the erosion in the subsequent etching step.
However, in plasma etching, the substrates are subjected to a temperature higher than 200° C. The photoresists containing as the base a novolak resin and a stabilizing improver cannot be thermally stabilized at a temperature of not lower than 180° C.
Polyimide resins are resistant to high temperature of about 400° C. and are stable to chemicals. Therefore, they are useful in forming heat-resisting photoresist layers.
Conventional polyimide photoresists are negative-type photoresists. The system of the negative-type photoresists is based on polyamic acid polymer having photoreactive side chains. However, this basic material has problems in that it has a poor storage stability, a very slow sensitizing rate, and an excess structural shrinkage after development and curing (the rate of shrinkage after baking is about 60%). With this composition, to attain a high resolution, exposure of about 10 minutes is necessary. Further, high concentration solutions thereof for forming thick films have especially poor storage stabilities (Tsuguo YAMAOKA et al., Polyfile 2, 14(1990).
It was shown that during the formation of the positive-type images, the polyimide composition having a weight average molecular weight of about 100,000 is dissolved in an alkaline solution and is converted to a composition having a weight average molecular weight of about 3000 (Takafumi FUKUSHIMA, Hiroshi ITATANI et al., Abstract of 49th Meeting of The Society of Polymer Science, Japan, (2000, Nagoya).